This invention relates to electrical power generation for power utilities and particularly to “islanding” of power plant generators.
Island control typically refers to the transition from parallel grid operation to isolated operation, otherwise known as island mode, and subsequent steady state island mode operation. Typically island mode operation is used to support relatively small local“house” loads. Parallel grid operation is typical of supplying power to an external electrical load. The transition to island mode occurs as a result of severing the tie line circuit breakers coupling the generator to the external electrical load during which the turbine remains in operation to support the local plant electrical loads. During the transition to island mode, the control system responds to the tie line breaker opening and enables the island speed control governor to automatically maintain the system frequency per the island speed set point.
Gas turbine island mode operability typically involves two stages: grid separation stage and island governor control stage. During the grid separation stage, the gas turbine undergoes load rejection. The sudden loss of load on the generator can cause the gas turbine to dramatically accelerate to over speed conditions. To counter the shaft acceleration and overspeed the speed governor, e.g., droop governor, responds by rapidly reducing fuel to limit the acceleration and avoid over speeding the gas turbine generator. The rapid reduction in fuel from the speed governor response imposes turbine operability restrictions during the grid separation stage. During the next stage, the island governor assumes control and regulates frequency to the island speed setpoint.
Power plants are often required to provide uninterrupted power generation after an unexpected disconnection from the electrical grid to support local electrical loads during the transient grid separation and beyond. The difference (“net load imbalance”) between local plant electrical load demand and the amount of power exported to the electric grid just prior to electrical grid separation dictates the electrical transient and gas turbine generator response during the grid separation stage. If the net load imbalance is large, the resultant gas turbine generator speed and acceleration response can be substantial. The resultant gas turbine generator response can determine the ability to support the local plant electrical load during the grid separation stage.
Conventionally, the same droop governor that regulates the gas turbine while operating parallel with the electrical grid is used to transition the gas turbine to island mode. A droop governor adjusts the fuel command of the gas turbine which drives the generator to maintain a desired frequency for the electrical grid. When disconnected from the grid, the droop governor responds to changes in the island frequency that occur as a result of the changes in the local load. The load and frequency changes that occur during the transition from grid to island mode operation may be quick and large. During this transition, the droop governor may not be able to fully respond to the changes. Further, the droop governor may not restore the generator frequency during island mode to a nominal frequency. Additional functionality, such as a preset and trim algorithm, have been added to a conventional droop governor to allow for correction and restoration of nominal frequency while in island governor control.
Upon grid separation, the gas turbine fuel governor reacts to the resulting shaft acceleration by rapidly reducing fuel to the combustors. The acceleration increases the airflow to the gas turbine. The fuel cutback coupled with the change in gas turbine airflow results in a transient combustor fuel/air mixture that may exceed the gas turbine Dry Low NOx operability design specification.
The traditional method to manage such transients has been to transition to a robust combustion operating mode which can support the rapid fuel and air changes during the transient. This conventional method limits the maximum power island load demand during the transient and requires, in some cases, significant load shedding locally within the plant. Alternatively, careful management of the gas turbine operation prior to the grid separation has been applied where plant operations limit plant export power thereby limiting the net load imbalance at the moment of grid disconnect. This conventional method can limit the maximum load attainable by the gas turbine generator during normal operation.
There is a long felt need for a gas turbine control system that provides improved island mode operation and transition to island mode. Further, there is also a long felt need for a control system that is not subject to some or all of the limitations of conventional control systems, such as those described above.